Step by step
The question
Question: find the height x of the house.
A 1.2 m girl has a 1.5 m shadow.
At the same time, the house has a 10 m shadow.
1 Two similar triangles
The sun's rays come in at the same angle.
So the two dashed lines are parallel.
The girl and the house both stand straight up: a right angle at the bottom.
Same angles, so the two triangles are similar.
One is a bigger copy of the other.
2 Match the sides
Matching sides go together.
The height goes with the height.
The shadow goes with the shadow.
3 Find the scale factor
The scale factor k says how many times bigger.
We know both shadows: 10 m and 1.5 m.
k is big over small.
Careful!
Never put a height with a shadow.
Shadow with shadow, height with height.
10 divided by 1.5 is 20/3.
The house's triangle is 20/3 times bigger.
4 Find x
x is in the big triangle.
Its partner is the girl's height, 1.2 m.
So x is 1.2 times 20/3.
That is 8.
So the height of the house is 8 m.
5 Check
Let's check: x with its partner.
8/1.2 is 20/3, the scale factor.
The girl is shorter than their shadow.
So the house is shorter than its shadow too.
It makes sense!
Let's look back.
First: two similar triangles.
Then: match the sides.
Then: find the scale factor.
Then: find x.
Last: check.